Update latest Solidus to v4

[waiting-for-dev]

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[kennyadsl]

@waiting-for-dev I think we need to change this line to make the CI work: https://github.com/solidusio/circleci-orbs-extensions/blob/master/solidus_dummy_extension/solidus_dummy_extension.gemspec#L19

[waiting-for-dev]

@waiting-for-dev I think we need to change this line to make the CI work: https://github.com/solidusio/circleci-orbs-extensions/blob/master/solidus_dummy_extension/solidus_dummy_extension.gemspec#L19

Weird, I had already done that, but probably I messed with git and removed the change thinking